1. Field of the Invention
The present invention relates to a power IC and an over-current protection circuit and method thereof. In particular, this invention relates to a power IC and an over-current protection circuit and method thereof that has a two-stage current limit protection mechanism.
2. Description of the Related Art
As technology develops, a variety of advanced electronic devices are produced. In addition to enhancing functionality of the circuits of the electronic devices, a great amount of efforts are exerted to the power circuit—the power IC, such as voltage regulators, that will affect the stability of the electronic devices.
The voltage regulator is a circuit that provides a constant voltage to the load. The output current of the voltage regulator is adjusted according to the resistance of the load so that the output voltage is maintained at a constant voltage. The characteristic of the voltage regulator depends on the electronic devices, such as the consumer electronic devices, or the portable electronic devices, etc. For example, low input-output voltage difference, high (low) output power, low quiescent current, low noise, or high power supply rejection to meet the requirements of the electronic devices. Therefore, in order to prevent the output current from being too large, or prevent the circuit from being damaged due to the output terminal is short-circuit, an over-current protection circuit is designed in the power IC so that the power IC is operated in a safe and stable status.
The power ICs with an over-current protection of the prior art can be divided into the following ways.
First, reference is made to FIG. 1, which shows a circuit diagram of the power IC with an over-current protection of the first way of the prior art. The over-current protection circuit 9 includes a current limit switch transistor Q1 and a sensing resistor R1. Because the output current flows through the sensing resistor R1, the resistance of the resistor R1 can be designed according the voltage over the sensing resistor R1. When the output current surpasses the specified value, the current-limit switch transistor Q1 is conducted to limit the output current. In other words, when the output current increases, the voltage over the sensing resistor R1 also increases so that the current limit switch transistor Q1 conducts current. Furthermore, the reference current source generates a bias current I1, and is connected with the collector terminal of the current-limit switch transistor Q1. Thereby, the driving current I2 flowing into the base terminal of the current limit switch transistor Q2 decreases. Therefore, when the output current surpasses the specified value, the output current is limited.
However, the over-current protection circuit 9 has two drawbacks. First, because the output current flows through the sensing resistor R1, the voltage over the sensing resistor R1 is too large when the output current is large, and a great amount power is lost on the sensing resistor R1. Therefore, there is a larger voltage difference between the input voltage VDD and the output voltage VOUT. Secondly, the over-current protection circuit 9 is sensitive for the temperature. Because the base-emitter voltage Vbe of the current limit switch transistor Q1 has a negative temperature coefficient and the sensing resistor R1 has a positive temperature coefficient, the default current limit threshold decreases due to the temperature increases.
Secondly, reference is made to FIG. 2, which shows a circuit diagram of the power IC with an over-current protection of the second way of the prior art. The power IC is composed of an over-current protection circuit 9′ and a voltage-regulating circuit. The voltage-regulating circuit includes an error amplifier EA, a power transistor M1, a feedback resistor net RF1 and RF2, and a reference voltage source VREF. When the load current of the output terminal of the voltage-regulating circuit increases (or decreases), the output voltage VOUT descends (or ascends). At this time, the feedback resistor net RF1 and RF2 outputs the variation of the output voltage VOUT to the input terminal of the error amplifier EA, and compares the output voltage VOUT with the reference voltage source VREF. Thereby, the error amplifier EA generates a control signal to control the magnitude of the biasing current I3 of the power transistor M1 to regulate the output voltage VOUT.
The over-current protection circuit 9′ includes a sensing transistor M2, a plurality of transistors M3, Q3, Q4, Q5, Q6, a reference current I4 and a capacitor C1. The current flowing through the power transistor M1, will generate a sensing current via the sensing transistor M2. The sensing current flows through the transistor Q4 and then is mapped to the transistor Q3. The reference current I4 provided by the current source is mapped to the transistor Q6 via the transistor Q5. The capacitor C1 is used as a compensation capacitor to prevent the collectors of the transistors Q3 and Q6 from generating an oscillation symptom. When the load current is too large and surpasses the current limit threshold, the current mapped to the transistor Q3 increases so that the voltage over the input voltage VDD and the point A increases to conduct the transistor M3 and drive the gate voltage of the power transistor M1 to a high level voltage. Thereby, the output current of the power transistor M1 is limited.
However, the over-current protection circuit 9′ has the following drawbacks. First, there is no fold-back current limit. When the output terminal is short-circuit, a great amount of heat loss occurs. In addition to wasting power, the power transistor M1 may be damaged when the voltage difference between the input voltage and the output voltage is large. Secondly, because the circuit needs a compensation capacitor, the area of the power IC is increased.